Phenolic resins are well known for their heat resistance. They tend to produce somewhat brittle, and nonresilient insulation coatings, however. Oil modified phenolics, while adding some flexibility, provide a structure which is difficult to control. Recognizing these problems, Mecum et al., in U.S. Pat. No. 3,061,579, provided a phenolic composition which included polyester linkages. The polyester component not only reduced the brittleness of the phenolic, but also added desirable properties, such as elasticity, structural strength, and adhesiveness. They provided much improved compositions compared to oil modified phenolics. In Mecum et al., the phenolic consisted of simple phenol reacted in excess with formaldehyde and cresol in the presence of sulfuric acid, to provide phenol Novolacs. The phenolic was then alcoholized to form an intermediate which was then esterified with dicarboxylic acid, such as azelaic acid, and dihydric alcohol, such as butane diol. The use of simple phenols alone, because of their trifunctional nature, produces very thick coating compositions that are difficult to apply for insulating motor coils and the like.
O'Donnell, in U.S. Pat. No. 3,216,884, used another approach to modifying phenolic resins. There, a phenolic-polyester impregnating emulsion was provided, comprising droplets of a solution of a phenol-aldehyde resin dissolved in a diluent, such as, at least one of water, alcohol, or ketone, suspended in an undissolved, unsaturated polyester resin. The weight ratio of phenolic:polyester was from about 1:1 to 20. The phenolic component can contain phenol, alkyl phenols where alkyl contains from 1 to 18 carbons, and their mixtures, i.e., substantially any phenol. Acid catalysts are most commonly employed, to provide phenolic Novolacs. This emulsion is used to impregnate cellulosic paper in the laminating art, where the diluent serves to initially swell the cellulosic fibers to increase their surface area, allowing uniform deposition of the diluent-insoluble polyester. Such emulsions, however, would be limited as insulating varnishes for motor coils and the like, because of their emulsion properties and their water base.
Payette, in U.S. Pat. No. 3,538,186, relating to magnet wire insulation, utilized a polyester base with 1 to 6 weight percent of a phenol-furfuraldehyde component to provide excellent flexibility, heat shock resistance, thermal life, and abrasion resistance properties. Here, the phenolic component could be any phenol, including substituted phenols and cresols, where reaction with furfuraldehyde proceeds in the presence of a basic catalyst, such as triethanolamine and a solvent such as cresylic acid. Laganis et al., in U.S. Pat. No. 4,261,873, relates to water soluble, oil free, fatty acid free polyester insulating varnishes, having high temperature properties and good hot bond strengths. There, 27 to 57 weight percent of the composition consisted of a phenolic resin reaction product of formaldehyde and a mixture of alkyl phenol, where alkyl contains from 1 to 12 carbons, polyhydroxyphenol, such as resorcinol, C.sub.6 H.sub.4 (OH).sub.2, and hydroxy benzoic acid.
While all of these resin systems solve various problems, there is still a need for a motor coil insulating varnish in a petroleum type solvent having low cost, ease of manufacture and application, and good electrical as well as good thermal and cure properties.